Doppler effect speed and drift indicating system



y 1 c. a. H. F-ELDMAN AL DOPPLER EFFECT SPEED AND DRIFT IND ICATINGSYSTEM Filed March 8, 1944 FIG. I

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' CBflFELDMA/V uvvavrog/s JWM? RAE Wat/hwy A om E y Patented July 12,1949 porrrina EFFECT s raan am mam mmcs'rma SYSTEM Carl B. H. Feldman,Rumson, N. 3., and James W. McRae, Alexandria, Va... assignors to BellTelephone Laboratories, Incorporated, New York, N. Y8, a corporation ofNew York Application March 8, 1944, Serial No. 525,554

3 Claims.

1 This invention relates to improved methods and apparatus fordetermining the speed and drift of aircraft with respect to the earthssurface. More particularly it relates to improved methods and apparatusfordetermining ground 5 speed and drift of aircraft by. use of theDoppler v effect.

The arrangements of the present invention represent an extension of theuse of principles disclosed in United States Patent 2,223,224, issuedNovember 26, 1940, to R. C. Newhouse. As ex? plained in this patent, ifa beam of energy of a particular predetermined frequency, preferably ofultra-high frequency radio wave energy (though any form of wave energycan obviously be employed), be radiated from an aircraft to strike thesurface of the earth obliquely, a part of it will be returned to theaircraft by the phenomenon known as difiuse reflection." Furthermore, ifthe aircraft is approaching, or receding from, the point of reflection,it is well known that because of the Doppler effect the apparentfrequency of the energy received on the aircraft will be greater. orless, than the frequency of the energy emitted therefrom, respectively.The frequency difference is further proportional to the instantaneousrate of change of the distance between the aircraft and the point atwhich reflection occurs. As explained in connection with Equation 2 ofthe above-mentioned Newhouse patent, the speed or velocity of theaircraft relative to any particular ray of the projected beam is afunction (cosine) of the angle of that ray with respect to the directionof motion of the aircraft. Thus it is apparent that the frequencydifference provides an index of the aircrafts speed with respect to thedirection of the point of reflection.

- In actual radio (or other forms of wave energy) systems, a sharppencil or single ray" beam is not readily obtained. In fact, withdirective radio antenna systems of reasonably small bulk a maximum beamWidth in the order of 30 degrees is not unusual. Therefore, assuming,byway of example, that the energy beam of a par- 45 ticularpredetermined frequency is being projected ahead of the craft at thesame azimuthal angle as that of the direction of motion of the craft,instead of the energy striking a single point on the earth it will bespread over an elliptical area. The speed of approach of the'craft withrespect to the several portions of this area will, obviously, differsomewhatbecause of the difierent vertical angles of the several rays ofthey beam projected from-the craft with respectto as the direction ofmotion of the craft. It is therefore also obvious that when thereflections of all the rays are receivedpn the aircraft any particularreflected ray will have an apparent frequency, as a result of thewell-known Doppler effect, which depends upon the vertical angle atwhich the particular ray was projected from the craft. That is to saythat a ray projected at the smallest vertical angle (and thus reflectedfrom the furtherest point on the above-mentioned elliptical area) willhave an apparent frequency substantially greater than that of .the rayprojected at the largest vertical angle. (The latter ray would,obviously, be reflected from the nearest point on the elliptical area.)Rays projected at vertical angles intermediate the above-describedmaximum and minimum angles would, of course,

vertical plane and a reasonably narrow horizontal, or azimuth, spread,to increase definition with -respect to azimuth angle variations, isdesirable,

since such a beam will strike an elliptical area having a long majoraxis along the azimuthal direction of the beam and a relatively shortminor axis, normal to the major axis. The shaping of horn antennas toobtain various types of fanshaped beams is discussed in detail in UnitedStates Patent 2,255,042, issued September 9, 1941, to W. L. Barrow. Aspointed out in Barrows patent the so-called sectoral horn, three speciesof which are illustrated in Figs. 1, 3'and 4 of the patent, is readilyproportioned, as described in detail in the patent, to provide a narrow(or highly directive) beam in the horizontal (X-Z) plane. In thevertical (X-Y) plane, the sectoral horn is also readily proportioned, asillustrated by the curves of Fig. 11 and discussed at page 6, column 1,starting at line 27, of the patent to provide a broad beam. It isfurther specifically pointed out in Barrows patent at page 6, column 1,starting at line 4, "By appropriate dimensioning the beam may be madefan-shaped.

. As explained in the above-mentioned patent to R. C. Newhouse, when theenergy beam has the same horizontal or azimuthal direction as that ofthe actual motion-of the craft, assuming the speed of the craft ismaintained substantially constant as the beam direction is changed, the

.apparent frequencyidiff r ces will be of maxirgies are detected,beat-note frequencies rep- :nting the frequency differences will beobied. The frequency p ctrum, or distribution,

hese frequency difference beat-notes will vary i the speed of theaircraft and with both the lzontal and vertical angles at which theenergy or is radiated with respect to the actual di- ;ion of motion ofthe aircraft. This follows :ctly from Equation 8 of the above-mentionedant to Newhouse.

a general, a more extensive frequency spec :n of beat-note frequenciesand particularly re of the higher beat-note frequencies will be ainedwhen the beam is directed at the same nuthal angle as the axis of actualmotion of craft, and the spectrum will contract and the her beat-notefrequencies will disappear prossively as the azimuthal angle of the beamis ned toward the perpendicular to the axis of ual motion, becoming aminimum when the nuthal direction of the beam is perpendicular reto.

.he next step, then, is to analyze the frequency tribution of beat-notefrequencies for various muth angles of the exploratory beam. The muthalposition for which the most extensive quency spectrum is obtained isindicative of azimuthal direction of actual motion of the craft and themaximum frequencies of the it-notes obtained for this beam position areindex of the actual speeds of the craft with pect to the earth.Similarly, the azimuthal for ich the least extensive frequency spectrumis 2ained indicates the line of the perpendicular the direction ofactual motion of the craft and an index of the drift or "crab angle ofthe craft with respect to its heading.

the principal objects of the invention are to wide improved methods andsystems for obning speed and drift indications for aircraft. her objectswill become apparent during the .11'58 of the following description andin the pended claims.

The application of the principles of the invenn will be more readilyunderstood from the de- Lled description of particular illustrativeemdiments given hereinafter, in conjunction with e accompanyingdrawings, in which:

Fig. 1 indicates in block schematic diagram rm one type of system forpracticing the prinoles of the present invention, the system being.apted to be mounted on an aircraft;

Fig. 2 indicates the synchronism of the oscilloope' sweep and thefrequency variation of the ating oscillator; Fig. 3 shows one form ofvariable rotary con- :nser and pulsing unit suitable for use with thestem of Fig. 1;

Figs. 4, 5 and 6 represent the change in the eat-note frequency spectrumwith change in :imuth angle of the exploratory beam of the stem of Fig.1 as shown upon an oscilloscope ith linear sweep; and

Fig. I shows a polar diagram of the beat-note equency spectrum variationfor the complete rotation in azimuth of the exploratory beam of thesystem of Fig. 1. v

In more detailin Fig. 1, the apparatus shown is assumed to be mountedon. an aircraft. The ultra-high frequency transmitter Iii energizes hornantenna ll through connecting transmission line II, to radiate andproject a beam of radio energy of a particular predeterminedfrequency,represented in part by rays 22, II and N, so as to strike the surface ofthe earth at an oblique angle. By the phenomena of diffuse reflection"(and by direct reflection where irregularities of the surface of theearth fortuitously present normal surfaces to the oblique rays of thebeam) some of the radiated energy, represented in part by rays 24, 02and 08, will be reflected back to receiving horn antenna 20 and will beconducted by transmission line 28 to receiving detectorv 80. By normal"crosstalk between horns i8 and 20 a small amount of the energy beingradiated will also be received in horn 20 and when combined with thereflected energy and detected in receiver-detector 30, 'a spectrum'ofbeat-note frequencies, arising by virtue of the Doppler effect upon thereflected energy, will be obtained, which will vary in character asdescribed above with changes in the azimuth angle of the projected beamwith respect to the actual direction of motion of the craft relative tothe earth. Should the normal crosstalk or coupling between the antennahorns be insuiflcient, small reflecting members as shown -in Fig. 2 ofUnited States Patent 2,193,361, issued March 12, 1940, to C. W. Rice, orthe equivalent, can be added to increase the coupling.

The frequency spectrum of beat-notes for each antenna direction can beanalyzed and examined by the familiar method of beating the spectrum.with an oscillator, the frequency of which is varied over a range suchthat at some frequency within the oscillator range each frequency of thebeat-note spectrum will produce a particular frequency which will passthrough a narrow bandpass fllter and the over-all distribution may beobserved on a cathode ray oscilloscope the ray of which is swepthorizontally in synchronism with the variation of the beating oscillatorfrequency as represented diagrammatically in Fig. 2, the ray beingdeflected vertically by passage of energy through the band-pass filter.Alternatively, the method of frequency analysis described in detail inUnited States Patent 2,084,760, issued June 22, 1937, to H. H. Beveragecan, obviously, be employed. This patent, incidentally, discloses alsothe use of a cathode ray indicator to display the relative amplitudes ofthe several frequencies of the particular spectrum being analyzed insubstantially the manner described above, though the type of frequencyspectrum being analyzed is of a distinctly different amplitudedistribution. i

The variation of frequency distribution with the angle of the horns i8,20 of Fig 1 is illustrated by curves 4 00, 500 and 600 of Figs. 4, 5 and6 respectively, which are representative of the indications which willbe provided by the system of Fig. 1 on oscilloscope 54 when switch 38 isin its upper position.

The spectrum of beat-note frequencies obtained .in receiver-detector 30is combined in modulator 32 with the output of oscillator 40, thefrequency of which oscillator is varied linearly in synchronism with thesaw-tooth. sweep wave of sweep generator 44 as indicated by curves 200and 202 of Fig. 2. The frequency of the oscillator is i yal'yfby e: thevariation of the] capacityof the variable rotary condenser-of unit ill..Th'edatter; unit includes a'cam operated springyswitch pulsingmechanism which provides Ea pulse-"for actuating the sweepgene'rator asasvaosa I granted my 31,1949, or,=alternatively', the, modulation of thesine and cosine waves'can be efas each frequency swee Starts. A suitableform for theiun'it l is shown in Fig.8 and will be described in detailin connection therewith.

.I'ilterllis a band filter passing a verynarrow lband offrequencies andthe frequency'r'ange of oscillator M is such each' frequency 'of thefected by the circuits described inv the abovementioned 'patent' of G.B. Engelhardt. I

' Themodulated sine and cosine waves are amplifled in'am'plifler 18* andone is applied to the horizontal deflecting plates 60 of oscilloscope4.6

ibeatnotepfrequency spectrum will, once each 'cycle, produce amodulation frequency in modulator 82 that will pus through filterv it.

-'1he output of filter 38 is rectified in rectifier I6 and applied, whenswitch 88 is in the upper Z position, to a vertical deflecting plate 58of oscillos'cope I. The output of sweep generator 84 is,

in parl' applied to the horizontal deflecting plate ll of oscilloscope Iand the combined action of the .above-described deflecting voltages willpro- 5 duce traces of the type shown as-curl/cs $00, 500

and continues. 4, band a, respectively. i

Curve tilt of'Fis. 4 can represent.-for example.

the broadest'spectrum of beat-note frequencies, 3. corresponding to themaximum speed of the 'air- Ecraft andthe coincidence of the direction ofhorns ll, "with the actual direction of motion g0! the aircraft, curve600 can represent some beveled gear i4. Beveled gear 26 meshes with:gear II and is carried on shaft M which can be rotated eitherby motor82 or by handwh'eel 60. g The right end of shaft it drives sine wavegener ator-ii which can be any conventional generator f of a reasonablytrue sine wave voltage, or alternatively, it can preferably be ofspecial form such as that disclosed in the copending application of EB.Morrison, Serial No. 492,263, filed June 25, 2 1943,-which issued onFebruary 25, 1947 as US. EPatent2,416',341, or that disclosed in thecopending application of G. B. Engelhardt, Serial No. 494,666, -fl1edJuly 14, 1943, which issued June 0, 1947as'U. 8 Patent 2,421,749, bothof which apfplications relate to sinewave generating means 1 peculiarlywell adapted for use in radar indicating circuits. Generator '88, inwhatever form may be chosen, should preferably provide two sine waveswhich are substantially identical but are in a quadrature relation, or,expressed in other words, a sine wave and a cosine wave of likeamplitude one'. ratio -so that the frequency of the sine iwavecorresponds to the rotational speed of the anter'niaaj Combiningcircuit 88 provides for modulating the sine andcosine-waves ofgenerator66 with the saw-toothwave of sweep generator 44. One

.. arrangement. for effecting the appropriate modu- 1 lationisshowmbyway of example, in United StatesiPatent 2,313,966 granted March1 16,- 1194.3 .to WJJ. Pooh. Conventional push-pull or-baland frequency;Gears It and 28 provide a one toand the other is applied to the verticaldeflecting plates 48 of'oscilloscope 68 so that the rayof theoscilloscope will rotate about the center point of its screenandsimultaneously will scan radially from the center "to the peripheryof the screen as for the so-callep. "plan position type of indication,now well knownin the art and disclosed, for example, in theabove-mentioned applications of Bryant and Engelhardt,

When it is desired to emplo'y the cathode ray oscilloscope 48, switch 88is thrown to its lower position'and the output ofrectifier 3B is thenapplied to the control anode 52 of-oscilloscope .46.

so-that as antennas I8, are rotated; the radially scanning rayofoscilloscop'e' lt will rotate in synchronism' therewith and the raywill be intensity modulated at eachangle in accordance with thefrequencyspectrum of beat-notes obtained for that-particular angle and apattern of radial :tra'ces as 102 shown in Fig. 7 will be obtained,

the brightened portions of the radii being of maximum length for thepositionof coincidence of the horns with the direction of actual motionI04 of the aircraft and decreasing to a minimum at the lineperpendicular to the direction of actual motion 106. Normally a smallopen circle about the center point of the oscilloscope screen will beobtained and indicates that the frequency range of the beatingoscillator is slightly morethan is required to "beat" with the lowestfrequency of the beat-note frequency spectrum to produce. a frequency topass through filter'ad.

- Similarly, the brightened radial traces of maximum length shouldpreferably not quite reach the out'erperiphery of the oscilloscopescreen, i. e.,

"the frequency range of beating oscillator should obviously be slightlygreater than required to bring both ends of the'maxlmum beat-notefrequency range upon the scale of the indicator. .As mentioned above theantennas i8, 20 are preferably of a type providing a fan-shaped beamwith 'it's'fbroaddimension in the vertical plane and are, as shown inFig. 1, directed toward the earth at an -angle of substantially45.degrees with respect to the horizontal plane, since such anarrangement will, obviously, provide a broader range of beat-notefrequencies This follows directly from Equation 8 of the above-mentionedPatent 2,223,224 to R, C. Newhouse'given at page 2, column 1, line 48,in which cos 0 appears as a .factor in determining the beat-note.frequency,

0 representing the angle of transmission below horizontal. With a broador fan-type beam, as above described, a spectrum of beat-note ranging infrequencybetween that obtaining for the value of 0 corresponding to theedge of the beam nearest perpendicular and that obtaining. for the valueof ocorresponding to the" edge of the beam near-- est horizontal will,manifestly,-be obtained.

The screen of oscilloscope 46' of Fig. 1. or

"of Fig. 7, can bear marks 108 to designate fore and aft and marks I Illto designateport and starboard, with respect to the heading of the craftv and the angle between marks I08 and'line 1M or between marks 110 andline is then the "crab angle" or angle between the actual direction ofmotion of the craft and its heading. A

scale indegrees, as illustrated to the'ieft of screen v the craft isflying. Such indications will, there- Hill in Fig. '7, is preferablyprovidedto facilitate fore, warn the pilot when the terrain beneathreading the "crab angle." The portion of the has. substantially changedits slope andyover scale below mark H obviously is applicable formountainous terrain will provide him with indidrift angles to the leftand that above mark H0 cations of the general slope of the terrainbeneath for drift angles to the right. 'As in the case of and willassist him in avoiding. collision by'driftthe indications obtained onoscilloscope 54 and ing off his true course. s

illustrated inFigs. 4, 5 andd, the maximumilength .In Fig. 2, asmentioned above,.saw-tooth 'curve ofra'dial; trace which isbrlglite nedonthe pattern 200 represents the sweep wave generated by gen-- of-Fig. 7is-a' measure of the speed ofthe aircraft l0 erator' 44 of Fig. 1 andemployed to provide the with respect to.the"surface oi'.the-earth and inhorizontal and. radial sweeps of oscilloscopes 54 I addition, as notedabove; its direction is that of and 48 of- Fig. 1, respectively. Curve 202 reprethe: actual direction of motion of the craft, -sents,. as abovementioned, the cyclic frequency l 'orfian airtra'itziiyingatasubstantiallyconvariation of the beating oscillator 40 ofFig. 1, ;stant'.';altitudegover reasonably leveiterrain the theoscillator passingthrough-its full range of maximum frequency beat-noteobtained on eitherfrequencies from"maximum .-.to minimum valuesindicator (1.: e.,' oscilloscope it or =54) will be a simultaneouslywith the; sweep voltage ofgenfunction"of': d. and.istherefore-a-',direct index; erator and; instantaneou'sly=returning toits; ofjthe'; 'spe'ed.-:",- A-I calibration scale in terms of jmaximumvalueto repeat itsfrequency variation 1-.speedeahtherefore,"obviously;bej directly assocyc n ync r n sm' w th thene t sw p wav .1ciated'with each ofthe}liftfithfilrsfiv a illustrated}cycle-w p j v in,1 ,'1ss:'4an i-'l;: o w lc the'speed-ean be read; a mm;3 the 'variableaq pacity unit and pulser.

i'f'he-scale divis'i'onsrequired-can be1computedfrom' whicheifectsitlre'c'yclic frequency variation of the t-t lief 1.-k ownations'relating to.'the Doppler". beatingfrequency ,c'ycle oscillator,and provides 'feifectgivem'for ar'n'ple, njthe abovegjm ntioned a pulse,at the start of each frequency cycle to 'atenttoR C.-.-Newho' e,leepai-ticularly Equa- "'synchr'onizethe saw-tooth i-wave generator 44.tioni- -8}giv h teaser; column; miners;- which 1 therewith is shown'and'comprises a conventional the-valueof.=theiangle Q'fo' the'j inaxfinh t... type of rotary variable capacitor which includes notes-frequency1 ,l opi; 1pi 1' *ghggggg; t t, two adjacent and opposed sets of spacedstator friearl'y horizontal ray; :th fl-shape'c iTibeam -39 plates3ilil'with-a-single set of spaced-rotor'plates 'an'idyiv;is thedesiredgro rid peed; or'mg m' i I 302 arranged to-interleave with thestator plates alternativel be ed- ,i;" fby" m i 1 1 t o I without makingphysical-contact therewith. 5 1 b fl' n m'e j timed e" Motor 308 servesto rotate-shaft3N uponwhich y ollstant' altitude' and o rotor 302 andcam 306 are mounted. As it rotates, xim hi b th 't r uenci s bbtain'ed-35.. cam 305 alternately forcesspring3l0 to the right number 'ofknownlspeed'siwithin the range and w t p rmi it o r vel to he left underrr-amass; I mg gi'ggali oz. w thhth fa g; its own spring tension foralternate halves of at d dqi tm ,mar j gs hag ggo u each-revolutionandcontact'springs -3l2 and 3H- ggo' sp jy w gm 'fi g are;thus.alternatelyconnected to terminal 332 Speed' M.= P.fH."illustrate-,an appropriat al f r 4 o h spring 310,- Spring 310 isforced to the oscillofspope, 5L In i q' t calibrationgmarky right by cam305 at thezmoment-rotor 302 passes naflzgj d no n bs ohvm fl completelyout of-the lower stator 300 and is .fo na'or-eir'cies.mutually,concentrlc about the m Tatum W Position a the left m;' 1 t cThey 5 the momentrotor 302 haspassed completely out ated zsoraooansflllgrespectively, andaceom 9 the upper t Terminals 334 are mniedxbyvmeJegen-d speed P. H, by way connected to the frequencydetermining-circuit of illustration; fdr'an mt u thebeatingoscillator'lfl'of Fig. 1 and the variable scgp e'lfi 1 Fromtheabdvfimenfioned Equations capacity of capacitor 300, 302 thus servesto vary of Newhous-e patent-1tis v that walk the oscillator frequencycyclically as required by a f a e b ished is-valid for an altitudes; h tof-mtor D st v h patternbf-jndications Pr f r bly shaped inacoordance'with principles tamed onwscmoscope 48 resulting fromavterraml well known in the art to provide a substantially which sldpessubstantiallyorimm a substantial 4 straight-line frequency variationwhen associated departure-from level night by: the aircraft; arewith-the frequency determining circuit of fortunately definite eds-flyrco-gnized; and 5 oscillator 40. Spring M0 is mechanicallyconreadily-interpreted character, as will be discussed nected to springby member which is b d speed indications are. obviously enof insulatingmaterial so that spring follows til-81y valid at; an flmesduflng which athe movements of spring 3). Springs SIG, 318 of is being. 0b 7constitute a, l n of t m a v I contactsighat liis, as spring 3i8 movesto the right obviou l t craft 1 v p ng ma escontact with spring 320before plane and a substantial (1111256512: ei z vgzg t fi breaks its ispnngals' similarly" relative radial. amplitudes of gthe fore and inwhen spring SIB-moves back to the leftfrom its lobes the pattern of mg.indicator 46 or extreme position-at-the rightit makes contact betweenthe port and starboard portions bf with spring 3i8 beforespring-3l8breaks contact the lobes isfobservedf-it will indicate that the I wlthspringmum-for each mo-vement-of craft s not ying in a plane whichissubstanspring P t lento its. right Positmn I any parallel. with the.emws surface. y s from its right to its left position, the circuit fromof the invention wili, therefore provide indicaterminal 328 terminalwhich includes tions of changesjin the relation of the plane of 2:3 3thi a t r r gg a fg 23% the'n3reli ht with res ct Sp i and 20'.beneath.i.-, This z:iay gf fiui r z fiigi fi r rg:- Ef 9 nect to sweepgenerator a changein' the angle of the plane of flight or vid r i fifrom battery as is thereby pm; from a change in anglevof the terrain v rwhi h 7 e eac time that cam 305 changes the posic a 1 of Springs 8 I0and-M8. As this is the instant aevaose at which a new frequency cycle isinitiated, the pulses to sweep generator 45 of Fig. I serve tosynchronize the saw-tooth wave sweep generator with the frequency cycle.The speed of rotation of motor 3% should be large with respect to thespeed of rotation of the antennas so that each sweep of the beat-notefrequency spectrum will correspond with the reflections received from asmall arc of the total circular area swept by a complete rotation of theantenna system.

In Figs. 4, 5 and 6, curves 00, 500 and 600, as mentioned above,represent the beat-note fre quency spectrum for three typical antennahorn directions, curve 3% corresponding, for example, to the directionof actual motion of the craft, curve 569 corresponding to some obliqueangle therewith and curve 690 corresponding to the perpendicular to theactual direction of motion. As noted above and as illustrated in Figs. 4and 7, scales reading speed directly can be associated with theoscilloscope and for the direction of actual motion of the craft thespeed with respect to the earth's surface can thenbe read directly fromthe scale, as illustrated at the left of oscilloscope screen Hill inFig. '7. Likewise, a scale from which the crab or drift angles of thecraft can be read directly is preferably provided.

In Fig. 7 the information gathered by continuously revolving the hornantennas is shown. The method and apparatus for obtaining thisindication and the character of the indication have been discussed indetail above.

Numerous variations and modifications of the above arrangements canreadily be made by those skilled in the art and systems employing othertypes of energy can be constructed without departing from the spirit andscope of the principles of the invention. By way of example, acompressional wave system for use on marine craft, obviously, could bedevised in accordance with the teachings of the invention to providespeed and drift indications with respect to the floor of the body of,water in which the craft is located. -The invention is defined-in thefollowing claims.

What is claimed is:

1. The method of detecting on an aircraft that the slope of the terrainbeneath the craft is changing appreciably which comprises radiating afan type beam of energy of substantially constant frequency in alldirections from said craft the broad dimension of said beam being in thevertical plane said beam being directed to strike the earths surfaceobliquely, rotating said beam about a vertical axis, receivingreflections of said energy beam on said craft, combining said receivedreflected energywith a small portion of the energy being radiated,detecting the combined energies to obtain a spectrum of beat-notefrequencies, and displaying the frequency distribution of said beat-notefrequency spectrum obtained for numerous directions of radiation,whereby changes in the relative frequency distribution of said spectrumfor various angles provides indications of change in the slope of the.

reflecting terrain.

derived and the received 2. An aircraft radio speed and drift indicatingsystem comprising, in combination, means for transmitting a fan-shapedbeam of radio energy of a particular predetermined frequency, the majoraxis of said beam being vertical, the axis of said beam being depressedfrom the horizontal at an oblique angle sufficient to cause the entirebeam to impinge upon the earths surface when said aircraft is proceedingparallel to said surface, means for receiving reflections of said beamfrom the earths surface, means for deriving energy directly from saidtransmitting means,.

means for combining said directly derived energy and said receivedreflected energy to obtain beatnotes representing frequency differencesbetween said transmitted and said received energy, means for rotatingsaid transmitting and receiving means about a vertical axis, means forfrequency analyzing the entire spectrum of beat frequencies obtained foreach azimuthal direction and indicating means cooperatively coupled withsaid rotating means and said frequency analyzing means for visuallyrepresenting the entire frequency spectrum of beat frequencies foreachand every azimuthal angle.

3. A navigational aid for mobile craft adapted for use on said craftwhen proceeding in a substantially horizontal plane with respect to theearths surface, said aid comprising in combination transmittingapparatus adapted to emit a fan-shaped beam of wave energy to strike theearths surface obliquely, said wave energy having a particularpredetermined frequency, the major axis of said fan-shaped beam being inthe vertical plane, receiving apparatus adapted to receive reflectionsof said beam from the earth's surface, apparatus adapted to derive asmall amount of wave energy directly from said transmitting apparatusand to combine the directly reflected wave energy to obtain beat-notesrepresenting apparent frequency differences between said directlyderived wave energy and said received reflected energy, beat-frequencyanalyzing apparatus adapted to isolate each frequency of the entirespectrum of beat frequencies obtained from said combination of waveenergies, means to rotate said beam in.

the horizontal plane and indicating apparatus adapted to visuallyindicate each and every beat frequency of the entire beat-frequencyspectrum being obtained at each instant.

. CARL B. H. FELDMAN.

JAMES W. McRAE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

